Conventional radio communications equipment are implemented primarily with analog circuitry. The inherent characteristics of analog components limit the amount of signal processing possible. For example, the noise and gain characteristics of analog amplifiers limit the dynamic range of the processed analog signal. In addition, analog information can not be readily stored in a manner which allows sophisticated signal processing.
The use of digital signal processing to replace operations previously performed using analog processing eliminates undesirable variations in those operations which may have resulted from external effects such as temperature, humidity, and aging on analog components. In addition, digital signal processing techniques offer flexibility in terms of programmable operating characteristics and features. For example, a digital intermediate frequency (IF) integrated circuit would be programmable in terms of its channel frequency, its sampling rate, and, to some extent, its filter response. A digital signal processor (DSP), executing alternate stored programs, can perform different filtering and demodulation to implement completely different types of radios. Also, the DSP may be used to introduce advanced processing techniques such as adaptive equalization.
An additional advantage of a digital receiver structure is that the DSP and IF circuitry can be designed so that it can be "reversed" to perform the corresponding operations for a digitally implemented transmitter. For half-duplex operation, the circuitry might be switched so that it simply reverses "direction," while for full-duplex operation two IF filters would be needed.
The primary technology contribution leading to the feasibility of a substantially digital receiver is a high-speed (20-100 MHz), high-resolution (10-12 bits) A/D converter. Clearly, before any digital processing can occur, the normally low level analog signal presented to the receiver must be converted into digital form.
The initial A/D conversion of the received signal presents several problems. In a land mobile system the magnitude of a received signal may be as low as 0.5 microvolts (uV). This signal level is significantly lower than the threshold sensitivity available in a conventional A/D converter; for example, a commercially available 1 volt 14 bit A/D converter has a threshold wherein the least significant bit (LSB) in its output corresponds to an analog input voltage of 61 uV. A substantial range (0.5 uV-60 uV) of input signal levels which is useful in conventional analog receivers, would not be detected by such an A/D converter and could not be processed by digital signal processing.
Of course, an amplifier could be utilized to amplify low level received signals prior to the A/D conversion. However, such an amplifier would likely give rise to severe intermodulation distortion in a land mobile receiver and would itself provide an intermodulation limit. If a stronger signal is present concurrently with a low level desired signal, the A/D quantizing noise will have a narrow frequency spectrum and can result in severe intermodulation distortion. By converting a received RF analog signal into digital form, the AD converter functions as a quantizer, that is, it functions to subdivide the analog signal into small but measurable increments.
The mathematical relationship between distortion and quantization step size is addressed in an article by W. R. Bennett entitled "Spectra of Quantized Signals" published in the Bell System Technical Journal, July 1948, pages 446-472.
In an article by Leonard Schuchman entitled "Dither Signals and Their Effect on Quantization Noise" published in the IEEE Transactions on Communication Technology, Dec. 1964, pages 162-165, the mathematical relationship between a dither signal and quantization noise is addressed.
Arthur Stephenson's article "Digitizing Multiple RF Signals Requires an Optimum Sampling Rate" published in Electronics, Mar. 27, 1972, pages 106-110, discloses a concept for utilizing an A/D converter, digital filter and digital demodulator for receiving and processing RF signals. The disclosed concept envisioned utilizing an automatic gain controlled amplifier to amplify the low level filtered RF signal prior to the A/D conversion.
In U.S. Pat. No. 3,816,831 to Leonard Mollod, the disclosed invention relates to the processing of Loran signals using hard-limiting techniques. RF noise is added to the input signal to maintain a desired-signal to noise ratio The combined signal and noise is amplified by a hard-limiting amplifier prior to the information decoding.
A digital integrating and auto-correlator apparatus is disclosed in U.S. Pat. No. 4,288,857 to Jack Wilterding and John Cozzens and is directed generally to signal-to-noise ratio enhancement. A signal containing noise and a separate reference noise signal are alternately coupled through an analog switch, a low pass filter, and a sample and hold circuit to an A/D converter.
A second major factor leading to the technical feasibility of a digital receiver structure is the high level of integration and high speeds attainable in VLSIIC implementations, ultimately permitting, for example, an all digital zero-IF selectivity section (DZ155).
Historically, intermediate frequency (IF) sections have been employed in transmitters and receivers to perform the major portion of a radio's selectivity since it may be technically difficult, or cost prohibitive, to develop sufficiently selective filters at the transmitted or received frequency. Transceivers may have more than one IF section. For example, some receivers employ two IF sections for recovering the transmitted information. These receivers are generally referred to as dual conversion receivers, whereas a single IF receiver would be referred to as a singular conversion receiver. Generally, any receiver with an intermediate frequency of zero Hertz is referred to as a direct conversion receiver.
Analog implementations of direct conversion receivers suffer from a variety of detriments including local oscillator (LO) radiation, which results from imperfect reverse isolation through the mixers, and may desense nearby receivers. Further, radio desense performance can be degraded by nonlinear effects in the mixers causing self-mixing of on-and off-channel signals which creates DC offsets and audio distortion. Also, in an application involving the reception of frequency modulated (FM) signals, the direct conversion analog receiver has no means of limiting the zero-IF signal. This causes unpredictable performance in fading and other adverse conditions.
The aforementioned dual conversion receiver alleviates some of the direct conversion problems. The additional isolation obtained by a dual conversion receiver solves the LO radiation problem. However, the solution is achieved at the cost of an additional mixer and local oscillator, in addition to a narrow band (generally crystal) filter to achieve the required isolation. Further, having a traditional IF section prior to the DC-IF section essentially band-limits the incoming signal to one channel. Thus, the self-mixing products caused by the nonlinear effects of the mixers generally will not fall into the passband of the filter in a dual conversion receiver.
Although the dual conversion receiver solves many of the problems experienced by the direct conversion receiver (although at additional cost and size requirements), the dual-conversion receiver experiences other detriments. As previously mentioned, the direct conversion FM receiver cannot limit the zero-IF signal. Thus, the use of unconventional detection methods are required. The typical solution to this problem is to up-convert the zero-IF signal to a third IF frequency where it can be limited and detected using conventional circuits. Up-converting requires another local oscillator, additional mixers and a summing circuit. Further, up-converting creates yet another problem. The quadrature paths in an analog receiver cannot be perfectly balanced for amplitude and phase characteristics because of the nonexact performance of the mixers and filters. Thus, a beat-note is (created due to imperfect cancellation in the summer) which degrades hum and noise performance and causes audio distortion. A proposed solution to this problem is to phase lock the L0 to an incoming pilot signal. This necessitates additional circuitry at the transmitter to transmit the pilot signal and also requires additional circuitry at the receiver to develop the phase lock loop and pilot filters. Lastly, the phase lock loop lock-time and pull-in range become critical receiver parameters.
Although the above discussion has concerned receivers, similar problems are experienced in transmitter IF sections, even though transmitter IF topologies are generally different than those employed in receivers. Generally, any analog implementation of an IF section will experience temperature and part-to-part variations that may degrade the IF section performance. Therefore, a need exists for an IF section that is insensitive to part and temperature variations and solves the above mentioned problems experienced in analog implementations of IF sections.
The present invention combines these new technologies with improved techniques for front-end analog processing and digital IF filtering to achieve a feasible design for a substantially digital receiver. The receiver structure of the present invention permits a revolutionary change in the manufacturing technology and operating characteristics of mobile radios. Furthermore, this approach permits a radio to be built with a minimal number of parts, which at once reduces parts and manufacturing costs, while also improving radio reliability and serviceabilty.